In the field of medical technology, various extracorporeal blood treatment devices are known that have an extracorporeal blood circulation system. The familiar extracorporeal blood treatment devices include for example dialysis machines and cell separators, both which necessitate access to the patient's vascular system. In the case of extracorporeal blood treatment, blood is removed from the patient with an arterial puncture needle via an arterial tube conduit, and is then returned to the patient via a venous tube conduit with a venous puncture needle.
Despite regular monitoring of the vessel access by hospital staff, there is a fundamental risk that the venous puncture needle could slip out of the patient's blood vessel unnoticed. If an arterial needle slips out, there may be an intake of air into the arterial tube conduit. If a venous needle slips out, however, this leads to the feared free flow of blood into the surrounding area. If it is not immediately noticed that the venous needle has slipped out, there is therefore a risk of the patient bleeding to death.
Various devices of different designs for monitoring the vessel access are known. In general, the known monitoring devices are based on the safety precautions which are standard in blood treatment appliances, and which trigger immediate interruption of the extracorporeal blood circulation in the event of incorrect vessel access.
WO 99/29356 A1 describes a monitoring device for a vessel access, in which the strength of an electrical current is measured which flows through the fluid in the tube conduit. For this, two electrodes are arranged upstream of the puncture needle, and these effect an electrical connection to the fluid in the conduit. This known monitoring device also envisages measuring the flow of electrical current between one of the electrodes and a further electrode that is placed on the patient's skin.
From US Publication No. 2004/0254513 A1, there is known an extracorporeal blood treatment device with an extracorporeal blood circulation system, which has a monitoring device for the arterial and venous vessel access. This known monitoring device has two electrodes, one of which is arranged on the arterial tube conduit and the other on the venous tube conduit, in order to produce an electrical connection between the fluid in the respective tube conduit and a monitoring unit, via two measurement lines. The monitoring unit measures the impedance between the two electrodes, with incorrect vessel access being deduced if the impedance does not lie within specified limits.
US Publication No. 2004/0254513 A1 also describes an application in infusion technology with just one tube conduit. The publication proposes providing a first electrode on the tube conduit upstream of the puncture needle to effect an electrical connection to the fluid flowing in the conduit, and providing a second electrode that is placed on the patient's skin. The monitoring unit is connected to both electrodes and monitors the strength of electrical current that flows from the first electrode on the tube conduit through the blood within the tube, the puncture needle, and that area of the patient's body between the puncture needle and the second electrode.
The devices described above have the disadvantage that producing the electrically conductive coupling points turns out to be relatively costly. Thus US Publication No. 2004/0254513 A1 in particular deals with the use of conductive polymers for producing the electrical connection to the blood in the blood tube. It is also a disadvantage that the known devices make it necessary to apply separate body electrodes.